My pool was just started up about 6 weeks ago and I have salt water, pebbletec, about 20,000 gallons, and I am still using about a gallon a week

Generators normally raise the pH. Adding the acid is normal.

I add about 4 cups a week

Its normal to add have to add acid for the first few months with a pebble finish. I have stonescape and have to add a 1/4 gallon about every week. Your SWCG will also have a demand for acid. It sounds normal.

Doublecheck your TC, raising it to the middle of the range may buy you a few more days between having to add it.

You add it as often as a test result indicates the need. don't get in th habit of assuming you so much every so often.